Pyrometry: A Practical Treatise on the Measurement of High Temperatures. Charles R. Darling

Pyrometry: A Practical Treatise on the Measurement of High Temperatures - Charles R. Darling


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directly or indirectly, from the gas thermometer. In the table on next page, a number of fixed points, determined by various observers, is given; the error, even at the highest temperatures, probably not exceeding ±2° C.

      In preparing the temperature scale of a pyrometer for practical use, the instrument is subjected successively to a number of the temperatures indicated in the table, and in this manner several fixed points are established on its scale. The space between these points is then suitably subdivided to represent intermediate temperatures.

      Table of Fixed Points.

Substance. Physical Condition. Deg. Deg.
Cent. Fahr.
Water (ice) At Melting Point 0 32
Water ” Boiling ” 100 212
Aniline ” ” ” 184 363
Naphthalene ” ” ” 218 424
Tin ” Melting ” 232 449
Lead ” ” ” 327 620
Zinc ” ” ” 419 786
Sulphur ” Boiling ” 445 833
Antimony ” Melting ” 631 1167
Aluminium ” ” ” 657 1214
Common Salt ” ” ” 800 1472
Silver (in air) ” ” ” 955 1751
Silver (free from oxygen) ” ” ” 962 1763
Gold ” ” ” 1064 1947
Copper (in air) ” ” ” 1064 1947
Copper (Graphite covered) ” ” ” 1084 1983
Iron (pure) ” ” ” 1520 2768
Palladium ” ” ” 1549 2820
Platinum ” ” ” 1755 3190

      It is necessary to point out that the figures given in the table refer only to pure substances, and that relatively small quantities of impurities may give rise to serious errors. The methods by which the physical condition to which the temperatures refer may be realised in practice will be described in the succeeding chapter.

      National Physical Laboratory Scale.—Exact agreement with regard to fixed points has not yet been arrived at in different countries, and an effort to co-ordinate the work of the National Physical Laboratory, the United States Bureau of Standards, and the Reichsanstalt, with a view to the formation of an international scale, was interrupted by the war. In 1916 the National Physical Laboratory adopted a set of fixed points on the Centigrade thermodynamic scale, in conformity with which all British pyrometers have since been standardised. It will be seen that the figures differ very slightly from those given in the previous table, which represent the average results of separate determinations in different countries.

      National Physical Laboratory Scale (1916)

Substance. Physical Condition. Deg. Deg.
Cent. Fahr.
Water (ice) At Melting Point 0 32
Water ” Boiling ” (760 mm.) 100 212
Naphthalene ” ” ” ” 217·9 424
Benzophenone ” ” ” ” 305·9 582
Zinc At Melting Point 419·4 787
Antimony ” ” ” 630 1166
Common Salt ” ” ” 801 1474
Silver (in reducing atmosphere) ” ” ” 961 1761
Gold ” ” ” 1063 1945
Copper (in reducing atmosphere) ” ” ” 1083 1982

      For higher temperatures the melting points of nickel (1452° C.) and palladium (1549° C.) are employed, but the accuracy in these cases is not so certain as with the substances named in the table. A useful point, intermediate between copper and nickel, has been established by E. Griffiths, and is obtained by heating nickel with an excess of graphite, when a well-defined eutectic is formed which freezes at 1330° C., or 2426° F.

      Temperatures above the Present Limit of the Gas Thermometer.—As it is not yet possible to compare an instrument directly with the gas thermometer above 1550° C., all higher temperatures must be arrived at by a process of extrapolation. By careful observation of a physical change at temperatures up to the limit of 1550° C., the law governing such change may be discovered; and


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